Field of the Invention
Embodiments of the present invention relate to a switching power supply apparatus in which power conversion efficiency has been improved.
Discussion of the Background
A resonance type converter is known as a switching power supply apparatus for use in various electronic equipment. The resonance type converter is constructed by connecting a primary winding of an insulation transformer to a DC voltage source via a capacitor. A DC resonance circuit is formed by a leakage inductor of the insulation transformer and the capacitor. The resonance type converter controls resonance current that flows through the DC resonance circuit balancing the first and second switching elements which are turned ON or OFF, and acquires a DC voltage which is stepped up/down from the secondary wiring side of the insulation transformer.
For example, U.S. Pat. No. 5,886,884 (“Patent Document 1”) and U.S. Pat. No. 7,391,194 (“Patent Document 2”) propose a soft switching technique in this type of switching power supply apparatus. This soft switching technique decreases loss in the switching element considerably by turning the switching element ON when voltage applied to each switching element is zero or when current that flows to the inductor is zero.
Referring to FIG. 11, in this resonance type switching power supply apparatus 1, the primary winding P1 of the insulation transformer T is connected to the DC voltage source B via the capacitor C, for example, and includes a DC resonance circuit formed by the leakage inductor of the insulation transformer T and the capacitor C. A first switching element Q1, which is connected in series to the primary winding P1 of the insulation transformer T, is turned ON by a drive control circuit A which is excited separately, and applies input voltage Vin from the DC voltage source B to the series resonance circuit. The drive control circuit A is a power supply IC, for example. A second switching element Q2, which is connected to the series resonance circuit in parallel, is turned ON by the drive control circuit A when the first switching element Q1 is OFF, and forms a resonance current path of the series resonance circuit. The first and second switching elements Q1 and Q2 are high withstand voltage n-type MOS-FETs, for example.
Power generated in the secondary windings S1 and S2 of the insulation transformer T is rectified and smoothed via an output circuit, which is constituted by diodes D1 and D2 and an output capacitor Cout, and is supplied to a load (not illustrated) as the output voltage Vout. A resonance type power converter main unit is constituted by these circuit units. The output voltage Vout, specifically the deviation of the output voltage Vout and an output voltage set value, is detected by an output voltage detection circuit VS, and is fed back to the drive control circuit A via a photocoupler PC as FB voltage.
The FB voltage fed back to the drive control circuit A is used for pulse width modulation of an output control signal which turns the first and second switching elements Q1 and Q2 ON/OFF, so as to stabilize the output voltage Vout. The DC power supplied from the DC voltage source B is normally filtered via the input capacitor Cin, and is then supplied as input voltage Vin to the switching power supply apparatus.
FIG. 12 shows the general configuration, where the drive control circuit A is comprised primarily of an output control circuit 2, a dead time circuit 3 and a drive signal generation circuit 4. The output control circuit 2 is constituted by a PWM control circuit that generates, as a PWM signal, an output control signal having a pulse width corresponding to the FB voltage which is fed back from the output voltage detection circuit VS, for example. Instead of the PWM signal, the output control circuit 2 may generate a pulse signal having a frequency in accordance with the FB voltage (PFM signal) as the output control signal.
If the output control signal is received, the dead time circuit 3 generates a dead time signal which turns the first and second switching elements Q1 and Q2 ON when the voltage applied to the first and second switching elements Q1 and Q2 is zero. The drive signal generation circuit 4 generates a drive signal of which pulse width is controlled to turn the first and second switching elements Q1 and Q2 ON in accordance with the dead time signal and the output control signal.
In FIG. 12, reference numeral 5 denotes a drive amplifier as a drive circuit that generates LO terminal output to drive the first switching element Q1 on the low side, in accordance with the drive signal outputted by the drive signal generation circuit 4. Reference numeral 6 denotes a drive amplifier as a drive circuit that generates HO terminal output to drive the second switching element Q2 on the high side by inputting the drive signal outputted by the drive signal generation circuit 4 to a level shift circuit 7. Reference numeral 8 denotes an internal power supply circuit that generates voltage VDD required for operation of the output control circuit 2, the dead time circuit 3 and the drive signal generation circuit 4, from the drive voltage VCC that is applied to the drive control circuit A.
Now the operation of the resonance type converter, which is the switching power supply apparatus having the above configuration, will be described in brief. In this resonance type converter, when the second switching element Q2 is in the OFF state, current flows to the series resonance circuit by turning the first switching element Q1 ON. If the first switching element Q1 is turned OFF in this state, a parasitic capacitor (not illustrated) of the first switching element Q1 is charged by the current flowing to the inductor of the series resonance circuit. At the same time, a parasitic capacitor (not illustrated) of the second switching element Q2 is discharged by this current.
By turning the second switching element Q2 ON when the charging voltage of the parasitic capacitor of the first switching element Q1 reaches the input voltage Vin, zero voltage switching of the second switching element Q2 is implemented. As the second switching element Q2 is turned ON here, power energy stored in the capacitor C flows through the second switching element Q2. As a result, the current that flows to the inductor of the series resonance circuit is inverted.
If the second switching element Q2 is turned OFF, the parasitic capacitor of the second switching element Q2 is charged by the above mentioned inverted current. At the same time, the parasitic capacitor of the first switching element Q1 is discharged by this current. By turning the first switching element Q1 ON when the voltage charged in the parasitic capacitor of the second switching element Q2 reaches zero voltage, zero voltage switching of the first switching element Q1 is implemented. By turning the first switching element Q1 ON like this, the current of the series resonance circuit is inverted and flows through the first switching element Q1 again.
The above mentioned dead time signal is used for specifying the turn ON timing of one of the first and second switching elements Q1 and Q2 based on the turn OFF timing of the other switching elements Q2 and Q1.